Polyesters are polymeric materials made from the esterification of polybasic organic acids with polyhydric acids. Perhaps the most commonly made and used polyester is polyethylene terephthalate (PET), which can be manufactured by reacting terephthalic acid with ethylene glycol.
Polyesters are currently being used in increasing amounts in various applications. For instance, polyesters are commonly used to make all types of containers such as beverage and food containers, photographic films, X-ray films, magnetic recording tapes, electrical insulation, surgical aids such as synthetic arteries, fabrics and other textile products, and other numerous items.
Because polyesters can be remelted and reformed, many efforts are underway to recycle as much polyester as possible after use. Before polyesters can be recycled, however, it is necessary to separate the post-consumer polyesters from other products and materials that may be found mixed with or coupled with the polyester. Unfortunately, many problems have been encountered in attempting to separate polyester from other waste materials. In particular, many prior art processes are not capable of efficiently or economically recovering polyester when a significant amount of impurities and contaminants are present. Most prior art processes for separating polyesters from other materials have been limited to floatation separation techniques and mechanical recovery processes.
In floatation separation techniques, polyesters are separated from other materials based on density differences. For instance, materials containing polyester can be combined with water, in which polyester is known to sink. The less dense materials, which float in water, can thus be easily separated from the submerged polyester. This procedure is relatively simple and very effective in separating polyesters from specific low density impurities. Floatation separation techniques, however, cannot be used if the polyester is found in combination with materials that sink in water or that have densities comparable to that of polyester.
For instance, post consumer polyester is typically mixed with polyvinyl chloride (PVC) and aluminum, which are not water buoyant. In fact, PVC has a density that is very similar to the density of PET and is often misidentified as PET. Both aluminum and PVC must be separated from polyester before it can be reused. In particular, if PET and PVC are remelted together, hydrochloric acid gases are produced which destroy the properties of the resulting plastic material.
In the past, in order to separate PET from PVC using a floatation separation technique in a specific gravity bath, others have attempted to modify the surface of the PVC so that the PVC will float in an aerated aqueous medium. For instance, in U.S. Pat. No. 5,234,110 to Kobler and U.S. Pat. No. 5,120,768 to Sisson various processes for separating PET from PVC flakes are disclosed. In these processes, the surface of the PVC flakes is treated in a manner so that the surface of the PVC is more likely to adhere to air bubbles when placed in an aqueous medium. In order for these processes to be efficient, however, the PVC flakes must have a high surface area to volume ratio. Consequently, the above processes are deficient in separating PVC chips from PET when the PVC chips have a large interior volume.
Besides failing to separate polyesters from heavier-than-water impurities, floatation separation techniques also fail to remove coatings that are commonly adhered to polyester. For example, polyester containers are commonly coated with vapor barrier coatings, saran coatings, and/or inks.
Mechanical recovery processes as used herein are washing processes used to strip specific binder and adhesive layers off polyester films without substantial reaction occurring between the polyester and the wash solution. For example, U.S. Pat. Nos. 5,286,463 and 5,366,998 both to Schwartz, Jr., both of which are incorporated herein by reference thereto, disclose a composition and process for removing adhesives, particularly polyvinylidene halide and polyvinyl halide based resins, from polyester films, such as photographic films. In one embodiment, the polyester films are mixed with a reducing sugar and a base to remove the adhesive polymeric resin from the film. An acid is then added to precipitate the resin, which can then be separated from the polyester film.
U.S. Pat. No. 4,602,046 to Buser et al. discloses a method for the recovery of polyester from scrap material such as photographic film having a polyester base and at least one layer of macromolecular organic polymer. Specifically, scrap material is cut or chopped into small individual pieces or flakes and treated in a caustic alkaline solution at a solids level of at least 25% by volume and under conditions of high shear. The organic polymer coating material is removed from the polyester flakes. The polyester flakes are then separated from the polymer coating material by filtration or centrifugation, rinsed in water, and dried. The recovered polyester flakes can be used as a feed stock for making films, bottles or other polyester articles. A method and apparatus for recovering silver and plastic from used film is also disclosed in U.S. Pat. No. 4,392,889 to Grout. In this method, the used film is first passed through a bath preferably comprising a hot caustic solution for precipitating silver layered on the film. The film then passes through a second bath of hot caustic until an adhesive sheet disposed on the film has been dissolved. Typically, the adhesive sheet is made of polyvinylidene chloride, which adheres the silver to the film. After a second caustic bath, the film is dried and available for use.
Other processes for recovering polyester from photographic films are disclosed in U.S. Pat. No. 3,928,253 to Thornton et al., U.S. Pat. No. 3,652,466 to Hittel et al., U.S. Pat. No. 3,647,422 to Wainer, and U.S. Pat. No. 3,873,314 to Woo et al.
As shown above, mechanical recovery processes have generally been limited to use with photographic films. In recycling the photographic films, silver is also recovered making the processes economically viable. Mechanical recovery processes, although very successful at removing the emulsion-type coatings found on photographic films, have generally not been successful in removing other types of coatings from polyesters. For instance, most of these processes are not capable of efficiently removing some of the vapor barrier coatings and inks that are applied to polyesters.
Other contaminants that are generally not removable from polyesters using floatation separation techniques and mechanical recovery processes as described above are entrained organic and inorganic compounds. These contaminants include, for instance, gasoline, kerosene, motor oil, toluene, pesticides and other compounds that are absorbed by polyesters when placed in contact therewith. If the entrained organic and inorganic compounds are not substantially removed from the polyester materials during recycling, the recycled polyesters cannot be used as food containers or as beverage containers.
Because of the above noted deficiencies in prior art processes, large amounts of recyclable polyesters are being scrapped and loaded into landfills or are being incinerated. Unfortunately, not only is the polyester not being reused, but the polyester materials are creating a waste management and disposal problem.
Recently, the focus of recovering polyester from the waste stream has changed from mechanical washing processes to chemically converting the polyester into usable chemical components. For instance, in U.S. Pat. No. 5,958,987, 6,147,129, and 6,197,838, all to Schwartz, Jr., which are incorporated herein by reference thereto, processes for recycling polyesters in which a portion of the polyesters are reduced to their original chemical reactants are disclosed. The processes include the steps of combining the polyester materials with an alkaline composition to form a mixture. The mixture is heated to a temperature sufficient to convert the surface of the polyester to an alkaline salt of a polybasic organic acid and a polyol.
The above described patents represent great advancements in the art. The process of the present invention is directed to further improvements in processes for recycling polyesters.